It is known in the art to provide vascular grafts as artificial vascular prostheses to be implanted in individuals with diseased blood vessels. For example, if an individual is suffering from atherosclerosis then a section of blood vessel may be replaced or bypassed with a vascular graft.
The problem with such vascular grafts is that they have a tendency to cause turbulence in the flow of the blood that they carry, particularly at the join between the vascular graft and the blood vessel at either end. This can result in plaque or tissue formation, reduced flow capacity and thrombosis, or occlusion in the blood vessel.
WO-A-00138591 discloses a vascular graft in which a tubular graft is provided with four equally spaced ridges on the interior of the graft. Each ridge is in the form of an axially extending helix. The ridges induce helical flow on the blood passing through the vascular graft. The provision of helical blood flow reduces the turbulence of the blood in the vascular graft which, in turn, reduces the likelihood of plaque formation, reduced flow capacity, thromboses or occlusion.
A problem with vascular grafts, in general, whether or not they have an internal helical formation is that while vascular grafts are usually flexible, they do not exactly replicate the physical properties of the blood vessel that they replace. Normally, the vascular graft will follow a smooth curve when it is bent. However, if it is bent excessively, either during implantation or due to the bending movement in vivo (for example, bending that occurs at a joint) then there is a risk that, instead of forming a smooth curve the graft will form a kink, effectively blocking the cross-section of the vascular graft. If a kink occurs while the vascular graft is implanted then this can be very dangerous as blood flow is significantly reduced or occluded.
One solution to this problem is disclosed in WO-A-20051092240. A vascular graft is disclosed which comprises an internal helical formation having a helix angle in the range of between 8° and 20° and which imparts helical flow on blood passing through the graft and an external helical formation, which has a helix angle greater than 50°, which is for supporting the vascular graft and preventing kinking of the graft. While this vascular graft works well and has been successfully implanted into patients, there are certain features of its construction which are sub-optimal. In particular, it has been found that the presence of both the internal helical formation and the external helical formation on the vascular graft can result in the graft being more stiff than is desired by surgeons implanting the graft. This is particularly so where the graft is for implantation from the groin of a patient to just above the knee of the patient (i.e. an above knee femoropopliteal bypass graft) as space for the implantation procedure is particularly limited for a surgeon at these sites.
The construction of the vascular graft in WO-A-2005/092240 is such that the internal helical formation is reinforced by an external deformation helix which sits in a helical groove in the exterior of the tubular graft. Where the external helical formation and the deformation helix intersect, they are fused together. One potential disadvantage with this arrangement is that if the external helical formation is damaged, for example during inadvertent mishandling during implantation, it could dislodge the deformation helix and thereby weaken the structural integrity of the internal helical formation.
The present invention seeks to alleviate one or more of the above problems and has arisen from the realisation that a better balance between the stiffness and flexibility of a vascular graft can be achieved by omitting the external helical formation from the vascular grafts in regions where the internal helical formation is present, thereby reducing the stiffness of the vascular graft without unduly risking kinking of the graft. While such a vascular graft is useful at all sites of implantation, it is particularly useful at implantation sites at anatomical locations, either where little or no bending of the graft is required (e.g. for an above knee femoropopliteal bypass graft), or where bending is normal, i.e. where the graft route is across a joint, for example across the knee in a below knee femoropopliteal bypass. Such grafts may also be beneficial for implantation at other grafts sites, such as, but not exclusively, vascular dialysis access grafts across the elbow, axillofemoral grafts and aortobifemoral grafts across the hip.